Systems, methods, and media for multiple reference arm spectral domain optical coherence tomography

ABSTRACT

In some embodiments, systems, methods, and media for multiple reference arm spectral domain optical coherence tomography are provided which, in some embodiments, includes: a sample arm coupled to a light source; a first reference arm having a first path length; a second reference arm having a longer second path length; a first optical coupler that combines light from the sample arm and the first reference arm; a second coupler that combines light from the sample arm and the second reference arm; and an optical switch comprising: a first input port coupled to the first optical coupler; a second input coupled to the second coupler via an optical waveguide that induces a delay at least equal to an acquisition time of an image sensor; and an output coupled to the image sensor.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based on, claims the benefit of, and claims priorityto U.S. Provisional Application No. 62/571,787, filed Oct. 12, 2017,which is hereby incorporated herein by reference in its entirety for allpurposes.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

N/A

BACKGROUND

Spectral domain optical coherence tomography (SD-OCT) is a type ofoptical coherence tomography that has become increasingly useful, asSD-OCT can generate data with relatively high sensitivity, high speed,and phase stability. However, conventional SD-OCT suffers from adepth-dependent sensitivity decay that decreases the utility of SD-OCTfor imaging certain types of samples. For example, such depth-dependentsensitivity decay can decrease the usefulness of SD-OCT for imagingtissues with irregular surface topology and/or large luminal organswhere the distance between the probe and the tissue surface often variesby a large amount.

In general, the sensitivity roll-off observed in SD-OCT systems isdetermined by the spectral resolution of the spectrometer used tocapture the spectral domain image data. The spectral resolution of thespectrometer is, in turn, limited by the finite size and number ofpixels in a linear detection array used to capture the image data.

Additionally, taking the Fourier transform of the real interferencesignal generated by an SD-OCT system leads to mirror images on bothsides of the zero-delay. This makes it more difficult to readilydistinguish between negative and positive image depths. To avoid thisambiguity, the zero delay is usually set at or outside the tissuesurface, as the empty space between an SD-OCT probe and the surfacegenerally does not generate much information. This makes it easier togenerate images of the tissue, but also reduces the depth sensitivity asthe empty space is sampled at high resolution, while tissue further fromthe surface is sampled at lower resolution, or not at all.

FIGS. 1A and 1B show examples 100 and 150 of conventional singlereference arm spectral domain optical coherence tomography systems. FIG.1A shows a representation of a widely used configuration of an SD-OCTsystem, which uses a Mach-Zehnder interferometer for SD-OCT. FIG. 1B isanother widely used configuration of an SD-OCT system which uses aMichelson interferometer for SD-OCT. In these conventional SD-OCTsystems, the imaging depth range is determined by both the number ofpixels and width of the pixels in a linear detector used to acquire thespectrum. Generally, the number of pixels are sufficient to achieverelatively longer imaging depth range, but the finite width of thedetector pixels limits the effective imaging depth range, as the widthof the detector pixels determine the width of the spectrum acquired byeach pixel. In general, the maximum imaging depth range is half thecoherence length of the light acquired by each pixel. Additionally, dueto conjugate symmetry, the zero-delay position is generally set outsideof the sample to avoid the ambiguity between mirror images in the realsignal. Accordingly, only half of the imaging range can generally beused for mirror artifact free images. Further, since the degree ofcoherence decays as a function of the path length difference, thesensitivity of an SD-OCT system rolls-off relatively rapidly and themeasurement sensitivity is not sufficient to image the sample close tothe edge of available imaging depth range.

As shown in FIG. 1A, a light source 102 can provide light to a samplearm and a reference arm via a polarization controller 104, an opticalamplifier 106, and a fiber coupler 108. A portion of light is directedtoward a sample arm (e.g., 80%), while a second portion of light isdirected toward the reference arm (e.g., 20%). An optical circulator110-1 directs light received from fiber coupler 108 toward a sample 112(in the sample arm), and a second optical circulator 110-2 directs lighttoward a reference reflector 114 (in the reference arm). Light in thesample arm can be directed toward the sample via collimating optics116-1 and focusing optics 118 (e.g., a lens), which can project a beamwith a depth of focus centered near the surface of sample 112. A portionof the beam can be reflected at various depths of the sample as afunction of reflectivity of the sample, which is then received byfocusing optics 118 and directed toward optical circulator 110-1 viacollimating optics 116-1, which directs the reflected light toward afiber coupler 120 to be combined with light from the reference arm.Collimating optics 116-2 direct a beam from the reference arm towardreference reflector 114, which reflects the beam back toward opticalcirculator 110-2 via collimating optics 116-2, and optical circulator110-2 directs the light reflected by reference reflector 114 towardfiber coupler 120 via a polarization controller 122 to be combined withlight from the sample arm. Fiber coupler 120 combines the light fromboth the sample arm and the reference arm, and directs the light towardspectrometer 124, which generates a wavelength dependent signalrepresenting the structure of the sample along the depth direction.

System 150 depicted in FIG. 1B operates using similar principles tosystem 100 depicted in FIG. 1A, but includes only a single opticalcirculator 110-3 before fiber coupler 108, rather than using an opticalcirculator in both the sample arm and reference arm. In both system 100and system 150, the length of the reference arm can be set by adjustingthe position of reference reflector 114 to set the depth of thezero-delay point with respect to sample 112.

Accordingly, new devices, systems, and methods for multiple referencearm spectral domain optical coherence tomography are desirable.

SUMMARY

In accordance with some embodiments of the disclosed subject matter,devices, systems, and methods for multiple reference arm spectral domainoptical coherence tomography are provided.

In accordance with some embodiments of the disclosed subject matter, asystem for spectral domain optical coherence tomography is provided, thesystem comprising: a light source; an image sensor; a sample arm coupledto the light source, wherein the sample arm is configured to cause lightfrom the light source to be projected toward a sample; a first referencearm having a first path length, wherein the first reference arm iscoupled to the light source and the sample arm; a second reference armhaving a second path length that is longer than the first path length,wherein the second reference arm is coupled to the light source and thesample arm; a first optical coupler configured to combine light from thesample arm and light from the first reference arm; a second opticalcoupler configured to combine light from the sample arm and light fromthe second reference arm; and an optical switch comprising: a first portcoupled to an output of the first optical coupler, a second port coupledto an output of the second optical coupler via a length of opticalwaveguide that induces a delay at least equal to an acquisition time ofthe image sensor, and a third port coupled to the image sensor, whereinthe optical switch is configured to selectively provide light from oneof the first port and the second port and block light from the other ofthe first port and the second port.

In some embodiments, the system further comprises: a processor that isprogrammed to: cause the optical switch to output light received by thefirst port during a first time window, cause the image sensor togenerate a first vector of data based on light received during the firsttime window, wherein the first vector of data represents a state of thesample in the first time window that includes a first time, cause theoptical switch to output light received by the second port during asecond time window, cause the image sensor to generate a second vectorof data based on light received during the second time window thatincludes a second time, wherein the second vector of data represents thestate of the sample in the first time window, cause the optical switchto output light received by the first port during a third time window,wherein the third time window is subsequent to the first time window andsecond time window and includes neither the first time nor the secondtime, and cause the image sensor to generate a third vector of databased on light received during the third time window, wherein the thirdvector of data represents a state of the sample in the third timewindow.

In some embodiments, the first vector and the second vector correspondto a first lateral position on a surface of the sample, and wherein thethird vector corresponds to a second lateral position on the surface ofthe sample.

In some embodiments, the processor is further programmed to generate amatrix of data, wherein the first vector is a first row of the matrix,the second vector is a second row of the matrix, and the third vector isa third row of the matrix.

In some embodiments, the first vector comprises at least N elements,wherein each of the N elements corresponds to a pixel of the imagesensor, and wherein the matrix comprises at least N columns.

In some embodiments, a second optical path from the second opticalcoupler to the second port of the optical switch is longer than a firstoptical path from the first optical coupler to the first port of theoptical switch by at least five light-microseconds.

In some embodiments, the second optical path from the second opticalcoupler to the second port of the optical switch is at least onekilometer long.

In some embodiments, the system further comprises a fiber splitter withan input coupled to the light source, a first output coupled to thesample arm, and a second output coupled to the first reference arm andthe second reference arm, wherein the fiber splitter is configured toprovide at least half of the light received at the input to the firstoutput.

In some embodiments, the fiber splitter is configured to provide threequarters of the light received at the input to the first output.

In some embodiments, the fiber splitter is a first fiber splitter, andthe system further comprises a second fiber splitter with an inputcoupled to the second output of the first fiber splitter, a first outputcoupled to the first reference arm, and a second output coupled to thesecond reference arm, wherein the second fiber splitter is configured toprovide half of the light received at the input to the first output.

In some embodiments, the system further comprises: a first opticalcirculator coupled to the light source, the sample arm, the firstoptical coupler, and the second optical coupler; a second opticalcirculator coupled to the light source, the first reference arm, and thefirst optical coupler; and a third optical circulator coupled to thelight source, the second reference arm, and the second optical coupler.

In some embodiments, the system further comprises a variable delay linecoupled between the light source and the first reference arm.

In some embodiments, a sensitivity of the system is at least 98 dB overthe entire imaging depth of the system.

In some embodiments, the second path length is longer than the firstpath length by half of the maximum imaging depth of the system.

In accordance with some embodiments of the disclosed subject matter, amethod for spectral domain optical coherence tomography is provided, themethod comprising: receiving, by a processor, a matrix of datacomprising: a first vector of data generated using a first reference armhaving a first path length and representing a state of a sample in afirst time window that includes a first time, a second vector of datagenerated using a second reference arm having a second path length andrepresenting the state of the sample in the first time window, a thirdvector of data generated using the first reference arm and representinga state of the sample in a second time window that does not include thefirst time, and a fourth vector of data generated using the secondreference arm and representing the state of the sample in the secondtime window; generating, by the processor, first image data based on thefirst vector of data and the third vector of data; generating, by theprocessor, second image data based on the second vector of data and thefourth vector of data; calculating, by the processor, a spatial offsetbetween the first image data and the second image data based on acomparison of a portion of the first image data and a portion of thesecond image data; appending, by the processor, a first plurality ofvectors to the portion of the first image data to generate a zero paddedimage, wherein the number of vectors appended is based on the spatialoffset; removing, by the processor, a second plurality of vectors fromthe second image data to generate a cropped image, wherein the number ofvectors removed is based on the spatial offset; and merging, by theprocessor, the zero padded image and the cropped image to generate animage of the sample with decreased sensitivity roll-off

In some embodiments, the first vector is a first row of the matrix andthe second vector is a second row of the matrix.

In some embodiments, the first vector comprises at least N elements,each of the N elements corresponds to a pixel of the image sensor, andthe matrix comprises at least N columns.

In some embodiments, the spatial offset corresponds to a particularnumber of columns of the N columns.

In some embodiments, each entry in the first vector corresponds to awavelength, and wherein the method further comprises mapping thewavelength for each element to a wavenumber k.

In some embodiments, the method further comprises performing a discreteFourier transform to convert each element of the first vector to a depthvalue.

In some embodiments, further comprising generating a real image of thesample based on the first image data by subdividing the matrix into twosubmatrices by selecting vectors on a positive side of a zero delay forinclusion in a first submatrix and selecting vectors on a negative sideof the zero delay for inclusion in a second submatrix.

In some embodiments, the portion of the first image data corresponds tothe first submatrix.

In some embodiments, further comprising generating a binary mask basedon the second submatrix.

In some embodiments, further comprising generating a weighting matrix Wcomprising a plurality of weighting coefficients Cm based on the binarymask and a function.

In some embodiments, the function is Cm=(tanh(x)+1)/2, where x variesfrom −2π to 2π.

In some embodiments, the first vector and the second vector correspondto a first lateral position on a surface of the sample, and the thirdvector and fourth vector correspond to a second lateral position on thesurface of the sample.

In accordance with some embodiments of the disclosed subject matter, anon-transitory computer readable medium containing computer executableinstructions that, when executed by a processor, cause the processor toperform a method for spectral domain optical coherence tomography isprovided, the method comprising: receiving, by a processor, a matrix ofdata comprising: a first vector of data generated using a firstreference arm having a first path length and representing a state of asample in a first time window that includes a first time, a secondvector of data generated using a second reference arm having a secondpath length and representing the state of the sample in the first timewindow, a third vector of data generated using the first reference armand representing a state of the sample in a second time window that doesnot include the first time, and a fourth vector of data generated usingthe second reference arm and representing the state of the sample in thesecond time window; generating, by the processor, first image data basedon the first vector of data and the third vector of data; generating, bythe processor, second image data based on the second vector of data andthe fourth vector of data; calculating, by the processor, a spatialoffset between the first image data and the second image data based on acomparison of a portion of the first image data and a portion of thesecond image data; appending, by the processor, a first plurality ofvectors to the portion of the first image data to generate a zero paddedimage, wherein the number of vectors appended is based on the spatialoffset; removing, by the processor, a second plurality of vectors fromthe second image data to generate a cropped image, wherein the number ofvectors removed is based on the spatial offset; and merging, by theprocessor, the zero padded image and the cropped image to generate animage of the sample with decreased sensitivity roll-off.

BRIEF DESCRIPTION OF THE DRAWINGS

Various objects, features, and advantages of the disclosed subjectmatter can be more fully appreciated with reference to the followingdetailed description of the disclosed subject matter when considered inconnection with the following drawings, in which like reference numeralsidentify like elements.

FIG. 1A shows an example of a conventional single reference arm spectraldomain optical coherence tomography system.

FIG. 1B shows another example of a conventional single reference armspectral domain optical coherence tomography system.

FIG. 2A shows an example of a system for multiple reference arm spectraldomain optical coherence tomography in accordance with some embodimentsof the disclosed subject matter.

FIG. 2B shows another example of a system for multiple reference armspectral domain optical coherence tomography in accordance with someembodiments of the disclosed subject matter.

FIG. 3 shows examples of images generated using multiple reference armsof a multiple reference arm spectral domain optical coherence tomographysystem implemented in accordance with some embodiments of the disclosedsubject matter.

FIG. 4 shows an example of a process for generating spectral domainoptical coherence tomography image data using multiple reference arms inaccordance with some embodiments of the disclosed subject matter.

FIG. 5 shows an example of a timing diagram representing collection ofimage data in accordance with some embodiments of the disclosed subjectmatter.

FIG. 6 shows an example of a process for synthesizing image datagenerated using different reference arms in accordance with someembodiments of the disclosed subject matter.

FIG. 7A shows examples of images representing various operations thatcan be performed during synthesis of image data generated usingdifferent reference arms in accordance with some embodiments of thedisclosed subject matter.

FIG. 7B shows additional examples of images representing variousoperations that can be performed during synthesis of image datagenerated using different reference arms in accordance with someembodiments of the disclosed subject matter.

FIG. 7C shows an example of weighted merging coefficients that can beused to synthesize image data generated using different reference armsin accordance with some embodiments of the disclosed subject matter.

FIG. 8 shows examples of sensitivity roll-off observed for a firstreference arm, a second reference, and a combination of the first andsecond reference arms of a spectral domain optical coherence tomographysystem implemented in accordance with some embodiments of the disclosedsubject matter.

FIG. 9 shows an example of hardware that can be used to implement animaging device and/or a computing device that can be used in connectionwith some embodiments of mechanisms for multiple reference arm spectraldomain optical coherence tomography implemented in accordance with someembodiments of the disclosed subject matter.

DETAILED DESCRIPTION

In accordance with some embodiments of the disclosed subject matter,mechanisms (which can include devices, systems, and methods) formultiple reference arm spectral domain optical coherence tomography areprovided.

In accordance with some embodiments of the disclosed subject matter, themechanisms described herein can be used to implement an SD-OCT systemwith reduced sensitivity roll-off (e.g., compared to conventional SD-OCTsystems) using multiple reference arm delays. In some embodiments, theoptical delay between multiple reference arms can be adjusted such thatthe most sensitive region for respective images generated using themultiple reference arms are set near the surface of a sample for a firstreference arm, and within the sample at about half the ranging depth ofthe first reference arm. In some embodiments, the mechanisms describedherein can use a fiber delay line to delay interferograms from onereference arm temporally such that interference originating from thesame sample location in the same temporal window can be detected by asingle detector. In some embodiments, the mechanisms described hereincan combine images obtained from multiple reference arm delays,producing a resultant image that has reduced sensitivity roll-off andextended imaging depth range.

As described below, an SD-OCT system implemented using mechanismsdescribed herein had a maximum sensitivity of >105 dB and a minimumsensitivity of 95 dB over a 6-mm ranging depth, and images of tissueacquired ex vivo demonstrates the capability of such a system to moreclearly visualize tissue at the edges of the ranging depth range.

In some embodiments, electromagnetic radiation reflected from a samplecan be split and combined with different reference beams havingdifferent path lengths. For example, the electromagnetic radiation inone reference arm can be delayed by half of the imaging range of thesystem with respect to the other reference arm, and can be combined withhalf of the electromagnetic radiation reflected from the sample togenerate an interference pattern. As another example, theelectromagnetic radiation from the second reference arm can be combinedwith the other half of the electromagnetic radiation reflected from thesecond to generate a separate interference pattern representing adifferent portion of the sample. In such examples, the two interferencepatterns can correspond to the back-scattered electromagnetic radiationfrom the sample interfered with reference electromagnetic radiation withtwo different path lengths.

In some embodiments, the path length (sometimes referred to herein asthe delay) of one or more reference arms can be adjusted using anoptical circulator that directs light from a light source toward areference mirror, with the position of the reference mirror beingadjustable. Additionally or alternatively, one or more reference armscan omit a reference mirror, and the reference beam can be provided bypassing light from the light source through optics with a path lengththat is about the same as the path length to the sample and back to thereference arm. For example, light from a light source can be splittoward two reference arms, with the reference arm light from a referencearm of fixed path length being directed to a fiber coupler (e.g., a 99/1fiber coupler), while another reference arm can be configured to have avariable path length that can be adjusted (e.g., by stretching thefiber, by increasing the length in free space, etc.).

In some embodiments, after combining electromagnetic radiation reflectedfrom the sample with electromagnetic radiation from the reference arms,and before passing the electromagnetic radiation corresponding to theinterference patterns created by the combination of sample armelectromagnetic radiation and reference arm electromagnetic radiation toan interferometer (e.g., via an optical switch), the combinedelectromagnetic radiation corresponding to one (or more) of thereference arms can be delayed by passing the electromagnetic radiationthrough a length of fiber optic waveguide (e.g., a single mode opticalfiber) to temporally delay the electromagnetic radiation with respect tothe combined electromagnetic radiation corresponding to anotherreference arm. For example, the length of the optical fiber can beconfigured to temporally delay the electromagnetic radiation from aparticular reference arm by an amount of time taken to acquireinformation from the electromagnetic radiation of the other referencearm by a linear detector.

In some embodiments, the spectrum of the interfered electromagneticradiation from each reference arm can be detected by dispersing theinterfered electromagnetic radiation as a function of wavelength in onedimension toward a detector array. For example, the dispersed signal canbe detected by a spectrometer or a line scan camera. Note that althoughlinear arrays of CCD pixels are generally described herein as being usedto detect such electromagnetic radiation, this is merely an example, andin some cases a portion of a two dimensional array can be used to detectsuch electromagnetic radiation.

In some embodiments, an optical switch can be used to alternately passthe interfered electromagnetic radiation from each reference armsequentially in order to alternately detect multiple interferedelectromagnetic signals that represent a sample in a particular timewindow. In some embodiments, the sequential detection of the spectra ofthe multiple interfered electromagnetic radiation corresponding to themultiple reference arms can ensure that acquired interference spectracorrespond to the same time window, as the electromagnetic radiationfrom one reference arm is temporally delayed by the same duration thatis used to detect each single spectrum.

In some embodiments, sequentially acquired spectra can be processed tocombine the information representing interference with light from eachreference arm. For example, the spectra can be processed by subtractingthe background, performing a fast Fourier transformation to generatedepth information (e.g., representing a reflectivity profile of samplein the axial/depth direction). In such an example, resultant depthinformation can be used to generate image data using the depthinformation corresponding to both of the reference arms. In someembodiments, using the mechanisms described herein can be used toproduce a resultant image that has reduced sensitivity roll-off andextended imaging depth range (e.g., compared to conventional SD-OCTsystems), which can mitigate one of the largest disadvantages ofconventional SD-OCT systems.

Various other schemes have been explored for mitigating the sensitivityroll-off of SD-OCT. For example, one scheme uses reference arm or samplearm phase shifting to introduce a carrier frequency to facilitatequadrature detection. The complex spectral interference signal from thequadrature detector allows the positive and negative delay signals to bemore easily distinguished, which facilitates placing the zero-delay inthe middle of the ranging depth, increasing the depth from which highresolution signals can be detected with higher sensitivity. However,such quadrature detection approaches typically require multipleadditional optical components that induce losses, which decreases theoverall maximum OCT sensitivity that can be achieved.

As another example, one scheme uses an optical switch in aninterferometer's reference arm to alternate the transmission of twodifferent reference arm delays. In such an example, a single referencearm can include multiple reflectors at different path lengths, and thelight in the reference arm can be alternately passed along each path.Interference spectra corresponding to each reference delay aresequentially acquired using a single spectrometer, and then combined byconcatenating the highest sensitivity portions of cropped imagesgenerated from each reference arm. While this approach can increasesensitivity at depth, because the images are acquired at differentpoints in time, transverse resolution can be expected to be reduced forimages obtained with high-speed scanning SD-OCT systems.

As yet another example, one scheme uses dual reference arms withcorresponding, separate interferometers with separate light sources andline scan cameras used to capture information from each reference arm.This scheme introduces additional complications (e.g., through theintroduction of different light sources, different sensors, etc.), andis also bulkier and more costly than schemes that use only a singlespectrometer.

As still another example, another scheme multiplexes multiplesequentially acquired spectra with slightly offset frequency combs toattempt to reduce sensitivity roll-off. While this approach can reducesensitivity roll-off, transverse resolution can be expected to bereduced for images obtained with SD-OCT systems due to the sequentialacquisition of data.

FIG. 2A shows an example 200 of a system for multiple reference armspectral domain optical coherence tomography in accordance with someembodiments of the disclosed subject matter. In some embodiments, system200 can include an electromagnetic source 202 (referred to herein aslight source 202 for convenience). Light source 202 can be any suitablelight source or light sources. For example, light source 202 can beimplemented using a superluminescent diode.

In some embodiments, light source 202 can emit electromagnetic radiation(referred to herein as “light” for convenience) to an optical amplifier206. Additionally, in some embodiments, a polarization controller 204can alter the polarization state of light emitted toward opticalamplifier 206, which can be used to affect the gain of optical amplifier206.

In some embodiments, light from light source 202 can be received by afiber splitter 208, which can be used to split the input light into twooutputs, a first output that can be directed toward a sample arm, and asecond output that can be directed toward multiple reference arms. Fibersplitter 208 can split the input light into any suitable portions. Forexample, fiber splitter 208 can be a 75/25 fiber splitter that directs75% of the input light toward the sample arm, and directs 25% of theinput light toward the reference arms.

In some embodiments, an optical circulator 210-1 can receive lightoutput from one port of fiber splitter 208, and can direct the light toa sample arm of system 200. In some embodiments, the sample arm caninclude optics 216-1 that can be used to direct a beam of light towardsample 112. In some embodiments, optics 216-1 can include any suitableoptics for generating a beam with a sufficient depth of focus togenerate data that can be used to generate optical coherence tomographysignals. In some embodiments, optics 216-1 can be configured to directlight toward a particular location. For example, optics 216-1 caninclude one or more components that can be actuated to change adirection of an output beam.

In some embodiments, a portion of the light directed toward sample 112light is reflected from sample 112 and received by optics 216-1, whichdirects the reflected light back toward optical circulator 210-1.Optical circulator 210-1 can direct light received from optics 216-1toward another fiber splitter 218. In some embodiments, fiber splitter218 can split the input light, and direct portions of the reflectedlight toward fiber couplers that can be used to combine the lightreflected from the sample with light from each reference arm. Fibersplitter 218 can split the input light into any suitable portions. Forexample, fiber splitter 218 can be a 50/50 fiber splitter (e.g., a 1×2or 2×2 3 dB fiber splitter) that directs 50% of the input light toward afirst fiber coupler associated with a first reference arm, and directs50% of the input light toward a second fiber coupler associated with asecond reference arm.

In some embodiments, the portion of light directed toward the referencearms by fiber splitter 208 can be received by another fiber splitter220, which can split the light again to direct a portion of light fromthe light source toward each reference arm. Fiber splitter 220 can splitthe input light into any suitable portions. For example, fiber splitter220 can be a 50/50 fiber splitter (e.g., a 1×2 or 2×2 3 dB fibersplitter) that directs 50% of the input light toward a first referencearm, and directs 50% of the input light toward a second reference arm.

In some embodiments, each output of fiber splitter 220 can be coupled toan optical circulator. For example, a first reference arm can include anoptical circulator 210-2, and a second reference arm can include anoptical circulator 210-3.

In some embodiments, each optical circulator (e.g., 210-2 and 210-3) candirect the light toward a reference reflector 214-1 and 214-2,respectively (e.g., via optics 216-2 and 216-3, respectively). Eachoptical circulator 210-2 and 210-3 can receive the light reflected bythe corresponding reference reflector 214-1 and 214-2, and can directthe reflected light toward a corresponding fiber coupler. For example,in some embodiments, optical circulator 210-2 can direct light reflectedby reference reflector 214-1 toward a first fiber coupler 222 that isalso configured to receive a portion of light reflected by sample 112(e.g., from fiber splitter 218). As another example, in someembodiments, optical circulator 210-3 can direct light reflected byreference reflector 214-2 toward a second fiber coupler 224 that is alsoconfigured to receive a portion of light reflected by sample 112 (e.g.,from fiber splitter 218). As shown in FIG. 2A, in some embodiments, apolarization controller 204 can be disposed between an opticalcirculator in a reference arm to control the polarization of lightdirected toward the corresponding fiber couplers. In some embodiments,fiber couplers 222 and 224 can be implemented using any suitable fibercoupler. For example, fiber coupler 222 (and/or fiber coupler 224) canbe a 99/1 fiber coupler that is configured to combine about half of thelight reflected from sample 112 with light reflected from referencereflector 214-1 (or from reference reflector 214-2).

In some embodiments, the optical path length of one or more of thereference arms can be controlled by positioning reference reflectors214-1 and/or 214-2 using any suitable technique. For example, in someembodiments, reference reflectors 214-1 and/or 214-2 can be associatedwith an actuator that can controllably position the reference reflector.In a more particular example, one of the reference reflectors (e.g.,reference reflector 214-1) can be positioned to provide a path lengthcorresponding to the path length along the sample arm to a surface ofsample 112, while another reference reflector (e.g., reference reflector214-2) can be positioned to provide a path length corresponding to thepath length along the sample arm to a point within sample 112 (e.g.,longer than half the maximum imaging depth range that can be realizedusing the other reference reflector).

In some embodiments, the combined light from each fiber coupler 222 and224 can be directed toward a separate port of an optical switch 230. Insome embodiments, a path from fiber coupler 222 to a first port ofoptical switch 230 can be a first length, while a path from fibercoupler 224 to a second port of optical switch 230 can be a secondlength. For example, as shown in FIG. 2A, the path from fiber coupler224 can include a length of single mode optic waveguide 226 that candelay the arrival of light from fiber coupler 224 to optical switch 230.

In some embodiments, a length of optic waveguide 226 can be configuredto delay light from fiber coupler 224 by an amount of time that a sensorof a spectrometer 124 uses to collect a single line of data. In suchembodiments, system 200 can control optical switch 230 to alternatelyprovide light from fiber coupler 222 and fiber coupler 224 tospectrometer 124 at a frequency based on the amount of time taken tocollect a single line of data. Due to the delay provided by opticwaveguide 226, the light provided from fiber coupler 224 can representthe state of the sample at substantially the same time that as thatrepresented by light provided from fiber coupler 222. Accordingly, thelight received sequentially by spectrometer 124 from fiber couplers 222and 224 can substantially correspond to the state of sample 112 duringthe same time window, which can facilitate imaging with greater depthpenetration, while rendering the imaging insensitive to motion of thesample that may otherwise cause artifacts in sequentially acquiredimages. Note that the output of fiber coupler 222 is sometimes referredto herein as channel 1, while the output of fiber coupler 224 issometimes referred to herein as channel 2.

In some embodiments, an image sensor of spectrometer 124 can be operatedin a line trigger mode that causes a line of data to be acquired on eachrising edge of a trigger signal (e.g., a TTL signal), or alternatively,on each falling edge of a trigger signal. In such embodiments, anothersignal can be used to control optical switch 230 such that data isalternately acquired from channel 1 and channel 2. For example, thesignal that controls optical switch 230 can have a frequency that issynchronous with the trigger signal, but has a lower frequency by half.In such an example, the optical switch control signal can rise or fallin a synchronous fashion with the trigger signal such that the opticalswitch control signal transitions at the same time that the triggersignal transitions. This synchronous operation can ensure the sequentialacquisition of data from channel 1 and channel 2 captures informationthat corresponds in time.

FIG. 2B shows another example 250 of a system for multiple reference armspectral domain optical coherence tomography in accordance with someembodiments of the disclosed subject matter. As shown in FIG. 2B, system250 can include reference arms that are implemented using a variabledelay line 252 in lieu of reference reflectors to create a path lengththat substantially corresponds to the path length to sample 112 alongthe sample arm. In such embodiments, several optical components can beeliminated, such as optical circulators 210-2 and 210-3 can beeliminated, optics 216-2 and 216-3, and reference reflectors 214-1 and214-2. In some embodiments, a variable component 254 can be included inone of the reference arms to facilitate control of the path length ofthe reference arm. For example, variable component 254 can be an air gapwith an adjustable length, a stretchable portion of fiber with anadjustable length, etc.

FIG. 3 shows examples of images generated using multiple reference armsof a multiple reference arm spectral domain optical coherence tomographysystem implemented in accordance with some embodiments of the disclosedsubject matter. As described in more detail below, an acquiredtwo-dimensional data matrix can include data from both channel 1 andchannel 2 in alternate columns (or rows), and OCT images can begenerated from the acquired data matrix by de-interleaving the data fromeach channel, removing a background component from each channel,remapping wavelength data to wavenumber data, performing a fast Fouriertransformation on the wavenumber data, and mixing of the imagesgenerated form each channel. For example, odd lines of the data matrixcan include spectra from channel 1, while even lines of the data matrixinclude spectra from channel 2 which is delayed with an offset (e.g., ofabout 3 millimeters (mm)). In such an example, the odd and even lines ofthe data matrix can be separated to produce separate imagescorresponding to channel 1 and channel 2, which can be processedseparately. In some embodiments, the mean of all spectra (e.g., in eachchannel) can be subtracted from each spectrum of the corresponding dataset for background removal. For example, spectra from channel 1 can beaveraged, and the average value can be subtracted from each value in thedata matrix corresponding to channel 1. Note that the mean can begenerated using the combined data, but the losses in the two channelsmay be different, causing a difference in the background value.

In some embodiments, after background removal, each spectrum can beremapped from a linear wavelength domain to a linear wavenumber domainusing a mapping function calibrated for the spectrometer. In someembodiments, the re-mapped spectra can be fast Fourier transformed togenerate a reflectivity profile in the depth domain.

As shown in FIG. 3, image 302 is a cross-sectional OCT image of a samplecorresponding to channel 1, while image 304 is a cross-sectional OCTimage of the sample corresponding to channel 2. As described above, dueto the longer path length of the reference arm corresponding to channel2, image 304 represents a portion of the sample that is deeper withinthe tissue than the portion represented in image 302 (although there issubstantial overlap). Additionally, the delay in channel 2 after thelight reflected from the sample is combined with the light reflectedfrom the reference reflector can cause the data in image 304 torepresent substantially the same time windows that are represented inimage 302.

In general, SD-OCT systems have larger sensitivity roll-off than sweptsource OCT (SS-OCT) systems, signal strength rapidly decays with opticaldelay in SD-OCT systems. This relatively rapid signal strength decay canbe observed in images 302 and 304. For example, in image 302 the signalstrength for tissue surface closer to an imaging probe surface (shown bysolid arrows pointing “up”) is stronger than that of tissue surface atlonger distance from the probe surface (shown by the solid arrowpointing more “down”). However, in image 304, which has an additionaloptical delay offset, the tissue surface located at longer distance fromthe image probe surface (shown by a solid asterisk) is relativelystronger than that same point in image 302. Note that the zero-opticaldelay was set just before the inner surface of the image probe wall whencapturing data used to generate image 302, which causes a separation inthe mirror images caused by the ambiguity between positive and negativedelays in SD-OCT data. Conversely, the zero-optical delay was set withinthe imaging probe surface when capturing data used to generate image304, and the mirror images consequently overlap in image 304.Accordingly, in some embodiments, information from image 302 can be usedto remove mirror image artifacts from image 304 prior to merging of theimages to generate image 306. Image 308 is a version of image 302 inCartesian coordinates, and image 310 is a version of image 306 inCartesian coordinates. As shown in FIG. 3, image 310 includes moreinformation at greater depths.

FIG. 4 shows an example 400 of a process for generating spectral domainoptical coherence tomography image data using multiple reference arms inaccordance with some embodiments of the disclosed subject matter. Asshown in FIG. 4, at 402, process 400 can include setting a firstreference arm zero optical delay to be near a surface of a sample (e.g.,an in-vivo tissue sample) to be imaged. For example, the zero opticaldelay can be set to be just outside the surface of the sample to preventformation of mirror artifacts in an OCT image generated using the firstreference arm. In some embodiments, the path length of the firstreference arm can be set using any suitable technique or combination oftechniques. For example, in some embodiments, the path length of thefirst reference arm can be fixed based on an expected distance between aprobe (e.g., including optics 216-1) and the tissue surface. As anotherexample, the path length of the first reference arm can be manuallyadjusted. In a more particular example, the path length can be set at aninitial value, and an operator can adjust the path length until arelatively small separation between mirror images is observed betweenthe positive and negative delay images generated based on the firstreference arm. As yet another example, the path length of the firstreference arm can be automatically adjusted to a value just before thesurface of the sample using any suitable technique or combination oftechniques, such as an active or passive range finding operation,analysis of data acquired using the first reference arm, etc.

At 404, process 400 can include setting a second reference arm zerooptical delay to be within the sample to be imaged. For example, thezero optical delay can be set to be about half the maximum imaging depththat can be achieved using the first reference arm. As the firstreference arm can be expected to have a zero optical delay at thetissues surface, data is generally only collected up to half of themaximum imaging depth that could be achieved if the zero optical delaywere set within the tissue (although this would result in an image withoverlapping mirror images). In some embodiments, the path length of thesecond reference arm can be set using any suitable technique orcombination of techniques. For example, in some embodiments, the pathlength of the second reference arm can be fixed based on an expectedimaging depth of the first reference arm (e.g., the path length can befixed at about 3 mm longer than the path length of the first referencearm). As another example, the path length of the second reference armcan be manually adjusted. In a more particular example, the path lengthcan be set at an initial value, and an operator can adjust the pathlength until a merged image (and/or an image produced using onlyinformation generated using the second reference arm) includes data fromdepths of the tissue that overlap a sufficient amount with the firstreference arm, while also including data from depths substantiallybeyond the maximum imaging depth of the first reference arm. As yetanother example, the path length of the second reference arm can beautomatically adjusted to provide a maximum imaging depth.

At 406, process 400 can provide light to a sample arm, the firstreference arm, and a second reference arm. For example, as describedabove in connection with FIG. 2A, light can be provided to the samplearm via a first fiber splitter (e.g., fiber splitter 208), while thelight can be provided to the first reference arm and second referencearm via two fiber splitters (e.g., fiber splitter 208 and fiber splitter220).

At 408, process 400 can operate an optical switch to provide lightreflected from the sample arm mixed with light reflected by the firstreference arm.

At 410, process 400 can record image data corresponding to a region neara surface of the sample at a time t_(n), based on the light receivedfrom the optical switch at 408. For example, as described above inconnection with FIG. 2A, at 410, process 400 can control an image sensor(e.g., a linear CCD or linear CMOS sensor) to capture data indicative ofdepths at which light was reflected by the sample based on interferencebetween light in the sample arm and light in the first reference arm.

At 412, process 400 can operate the optical switch to provide lightreflected from the sample arm mixed with light reflected by the secondreference arm. In some embodiments, process 400 can control the opticalswitch to account for the time used to acquire the image data from thefirst channel, and the delay in the second channel. For example, asdescribed below in connection with FIG. 5, the image sensor can beoperated using a trigger signal with a frequency having a period that issubstantially the same as the time the linear sensor uses to acquire asingle line of data.

At 414, process 400 can record image data corresponding to a regionwithin the sample at time t_(r), based on the light received from theoptical switch at 412. For example, as described above in connectionwith FIG. 2A, at 414, process 400 can control an image sensor (e.g., alinear CCD or linear CMOS sensor) to capture data indicative of depthsat which light was reflected by the sample based on interference betweenlight in the sample arm and light in the second reference arm. In someembodiments, process 400 can return to 408 to switch back to the firstchannel and acquire data, at 410, corresponding to time t_(n+1).

FIG. 5 shows an example 500 of a timing diagram representing collectionof image data in accordance with some embodiments of the disclosedsubject matter. As shown in FIG. 5, a trigger signal that is used tooperate the linear sensor of the spectrometer can have a firstfrequency. In the example shown in FIG. 5, the linear sensor isconfigured to capture data in response to detecting the rising edge ofthe trigger signal. An optical switch control signal can have afrequency that is half of the frequency of the trigger signal to controlthe optical switch to outputs light from channel 1 and channel 2 duringconsecutive data acquisitions by the linear sensor. Note that, in somecases, the acquisition time of the sensor may occupy a significantportion of the period of the trigger signal. For example, as shown inFIG. 5, if readout is triggered by the rising edge, the acquisition bythe sensor may not be completed until after the corresponding fallingedge of the trigger signal.

FIG. 6 shows an example 600 of a process for synthesizing image datagenerated using different reference arms in accordance with someembodiments of the disclosed subject matter. At 602, process 600 canreceive interleaved image data corresponding to data receivedsequentially from channel 1 and channel 2 of an imaging system. In someembodiments, each row or column of the interleaved image data cancorrespond to an A scan of a sample, and pairs of consecutive columnscan correspond to A scans of the same lateral location capturedsimultaneously. As described above in connection with FIG. 2A, in someembodiments, one of the channels can be associated with a delay suchthat consecutive spectra represent the state of the sample during thesame time window, but having different sensitivities at different depths(e.g., the delayed reference arm can be longer, which can facilitateimproved sensitivity for larger depth values). In some embodiments, theinterleaved image data can be formatted in the wavelength domain, inwhich each column or row corresponds to a depth value. Alternatively, insome embodiments, the interleaved image data can be received in anotherformat, such as in the wavenumber domain.

In some embodiments, the interleaved image data can be described as adata matrix representing M interference spectra, where each of the Minterface spectra is associated with N samples (e.g., representinguniform intervals of spectrum corresponding to a range of depth). Forexample, the interleaved image data can be represented as I(λ, t)_(m×N).

At 604, process 600 can separate the odd and even rows (or columns) togenerate separate data matrices corresponding to information receivedvia channel 1 and channel 2, respectively. For example, the matrixI(λ,t)_(M×N) can be separated into two matrices I_(Ch1)(λ,t)_(M/2×N) andI_(Ch2)(λ,t)_(M/2×N), each representing an image of the samplecorresponding to a different reference signal.

At 606, process 600 can remove a background component from the channel 1image by calculating the average (i.e., mean) value of each value in thematrix I_(Ch1)(λ,t)_(M/2×N), and uniformly subtracting the mean from thedata. For example, process 600 can generate an adjusted matrixI′_(Ch1)(λ,t)_(M/2×N) that corresponds to I_(Ch1)(λ,t)_(M/2×N) with abackground component removed, which can be represented asI′_(Ch1)(λ,t)_(M/2×N)=I_(Ch1)(λ,t)_(M/2×N)−rep{meanI_(Ch1)(λ,t)_(1×N)}_(M/2×N),where meanI_(Ch1)(λ,t)_(1×N) is a column vector of dimension N whereeach element is the mean value of all elements in the row of matrixI_(Ch1)(λ,t)_(M/2×N), and rep{ } transforms the mean vector(meanI_(Ch1)(λ,t)_(1×N)) to a matrix of repeated similar mean vectorcolumns.

As 608, process 600 can map the wavelengths represented in the adjustedmatrix I′_(Ch1)(λ,t)_(M/2×N) to the wavenumber domain (sometimesreferred to as the k-domain). For example, the center wavelength λ_(N)corresponding to each column can be mapped to a wavenumber k_(N) (notethat in general wavenumber corresponds to the inverse of frequency,i.e.,

$\left. {k_{N} = \frac{1}{\lambda_{N}}} \right).$

The mapping can be represented asI′_(Ch1)(λ,T)_(M/2×N)→I_(Ch1)(k,t)_(M/2×N). In some embodiments, themapping function can be calibrated for the spectrometer with linearinterpolation.

At 610, process 600 can calculate a discrete Fourier transform (DFT) ofthe k-domain data matrix corresponding to channel 1 to generate arepresentation of the channel 1 image in the spatial domain. Forexample, process 600 can perform a discrete Fourier transform of thek-domain data matrix to generate a spatial domain data matrixI_(Ch1)(z,t)_(M/2×N) representing the sample at various depth valuesz_(N). In a more particular example, process 600 can perform a fastFourier transform of the k-domain data matrix, which can be representedas FFT{I_(Ch1)(k, t)_(M/2×n)}→I_(Ch1)(z,t)_(M/2×N).

At 612, process 600 can split the channel 1 image (e.g., thespatial-domain data matrix) to separate mirror images in the channel 1image due to ambiguity between the positive and negative delay. Forexample, process 600 can subdivide the channel 1 image by using thefirst N/2 columns (or rows) to represent one version of the mirrorimage, and using the next N/2 columns (or rows) to represent anotherversion of the mirror image. In a more particular example, process 600can generate a data matrices I′_(Ch1)(z,t)_(M/2×N/2) andI_(Ch1)(z,t)_(M/2×N/2), where data matrix I′_(Ch1)(z,t)_(M/2×N/2) isbased on the 1st to (N/2)^(th) columns of I_(Ch1)(z,t)_(M/2×N) and datamatrix I_(Ch1)(z,t)_(M/2×N/2) is based on the (N/2+1)^(th) to Nth columnof I_(Ch1)(z,t)_(M/2×N). Note that, in some embodiments, process 600 canomit generation of one of the mirror images when only one of the mirrorimages is used in later processing.

At 614, process 600 can remove a background component from the channel 2image by calculating the average (i.e., mean) value of each value in thematrix I_(Ch2)(λ,t)_(M/2×N), and uniformly subtracting the mean from thedata. For example, process 600 can generate an adjusted matrixI′_(Ch2)(λ,t)_(M/2×N) that corresponds to I_(Ch2)(λ,t)_(M/2×N) with abackground component removed, which can be represented asI′_(Ch2)(λ,t)_(M/2×N)=I_(Ch2)(λ,t)_(M/2×N)−rep{I_(ch1)(λ,t)_(1×N)}_(M/2×N),where meanI_(Ch2)(λ,t)_(1×N) is a column vector of dimension N whereeach element is the mean value of all elements in the row of matrixI_(Ch2)(λ,t)_(M/2×N), and rep{ } transforms the mean vector(meanI_(Ch2)(λ,t)_(1×N)) to a matrix of repeated similar mean vectorcolumns.

As 616, process 600 can map the wavelengths represented in the adjustedmatrix I′_(Ch2)(λ,t)_(M/2×N) to the wavenumber domain (sometimesreferred to as the k-domain). For example, the center wavelength λ_(N)corresponding to each column can be mapped to a wavenumber k_(N) (notethat in general wavenumber corresponds to the inverse of frequency,i.e.,

$\left. {k_{N} = \frac{1}{\lambda_{N}}} \right).$

The mapping can be represented asI′_(Ch2)(λ,t)_(M/2×N)→I_(Ch2)(k,t)_(N/2×N). In some embodiments, themapping function can be calibrated for the spectrometer with linearinterpolation.

At 618, process 600 can calculate a discrete Fourier transform (DFT) ofthe k-domain data matrix corresponding to channel 2 to generate arepresentation of the channel 2 image in the spatial domain. Forexample, process 600 can perform a discrete Fourier transform to thek-domain data matrix to generate a spatial domain data matrixI_(Ch2)(z,t)_(M/2×N) representing the sample at various depth valuesz_(N). In a more particular example, process 600 can perform a fastFourier transform of the k-domain data matrix, which can be representedas FFT{I_(Ch2)(k,t)_(M/2×N)}→I_(ch2)(z,t)_(M/2×N).

At 620, process 600 can use one of the split images generated at 612 andthe image generated at 618 to determine a depth offset z_(shift) betweenthe two channels, which corresponds to the difference in path lengthbetween the two reference arms. In some embodiments, process 600 can useany suitable technique or combination of techniques to determine thez_(shift) between the two channels. For example, process 600 can use across-correlation function to determine which columns in the mirrorimage from channel 1 correspond to columns in the image from channel 2.In some embodiments, the spatial offset can be calculated once, as theoffset generally remains fixed. However, in some embodiments, the offsetcan be recalculated if the path length of one of the reference armschanges, and/or after a predetermined period of time and/or after apredetermined number of images have been captured (e.g., as the pathlength may drift over time due to environmental factors, such astemperature, etc.).

At 622, process 600 can zero pad the mirror image extracted at 612 basedon the spatial offset z_(shift) calculated at 620. This can cause thedepth of the image extracted at 612 to substantially match the maximumdepth of the image generated from channel 2. In some embodiments, thezero padding can be represented asI_(Ch1)(z,t)_(M/2×N/2)→I_(Ch1)(z,t)_(M/2×N)′ to match the dimension ofan image I_(Ch2)(z,t)_(M/2×N)′ generated from the channel 2 image basedon the offset (e.g., as described below in connection with 624).

At 624, process 600 can crop the image generated at 618 based on thespatial offset to remove a portion of the mirror image that is notrepresented in the image from channel 1. For example, process 600 cangenerate an image I_(Ch2)(z,t)_(M/2×N)′=I_(Ch2)(z,t)_(M/2×(N−z) _(shift)₎.

At 626, process 600 can generate a mask B that represents a binarizedportion I′_(Ch2)(k,t)_(M/2×N/2) of the cropped image generated at 624,which corresponds to a portion that overlaps with the zero padded imagefrom channel 1 generated at 622. In some embodiments, mask B can begenerated based on a comparison of the mirror image of channel 1 withthe main (e.g., positive delay) image from channel 2. For example, thevalue of each element in the mirror image of channel 1 can be comparedwith a threshold value. If the element has a value above the threshold a0 can be included in the binary mask B at that position, and if theelement has a value below (or equal to) the threshold a 1 can beincluded in the binary mask B at that position. In a more particularexample, the threshold can be based on the noise value (e.g., mean andstandard deviation) from a signal free region. For example, in 706,portions of the matrix within zero delay and imaging probe surface thathas no reflected signal can be used to determine the threshold.

At 628, process 600 can calculate merging coefficients that can be usedto merge overlapping data from the data matrices representing channel 1and channel 2. In some embodiments, process 600 can calculate a weightedmerge coefficient matrix W=Cm*B, where Cm is a merge coefficient. Forexample, Cm can be a coefficient that can be represented asCm=(tanh(x)+1)/2, where x varies from −2π to 2π, and can be used togenerate a vector of values for multiplication with mask B. Note thatthis is merely an example, and other functions, such as a linearlyvarying function, a step function, etc., can be used to generate theweighting coefficients.

At 630, process 600 can generate a merged image I_(m)(z,x)_(M/2×N)′using the zero padded mirror image generated at 622 (corresponding tochannel 1) and the cropped image generated at 624 using the mergingcoefficients (e.g., based on matrix W). In some embodiments, process 600can use any suitable technique or combination of techniques to merge theimages from channel 1 and channel 2. For example, I_(m)(z,x)_(M/2×N)′can be generated using the following formula to generate the portionthat is represented in both channel 1 and channel 2 images:

I _(m)(z,x)_(M/2×(N−Z) _(shift) ₎=(1−W)*I _(Ch1)(z,t)_(M/2×(N−z)_(shift) ₎ +W*I _(Ch2)(z,t)_(M/2×(N−Z) _(shift) ₎

In some embodiments, after generating a merged image, process 600 cancause the image to be stored (e.g., in memory, in a cache, in long termstorage, etc.) and/or presented (e.g., using a display).

FIG. 7A shows examples of images representing various operations thatcan be performed during synthesis of image data generated usingdifferent reference arms in accordance with some embodiments of thedisclosed subject matter. As shown in FIG. 7A, image 702 represents oddcolumns (or rows) of the 2D data matrix output by the spectrometer,while image 704 represents even columns (or rows) of the 2D data matrixoutput by the spectrometer, resulting in images corresponding to channel1 and channel 2 after background subtraction. Note that this is merelyan example, and channel 2 can correspond to the even columns (or rows).The spatial difference between the channel 1 and channel 2 images can beobserved by visually comparing images 702 and 704.

In FIG. 7A, image 706 represents a mirror image of the tissue (e.g.,rather than a real image of the tissue), which can be determined becausethe zero delay is set just outside the imaging probe wall, andconsequently, the negative delay space is occupied by mostly empty space(i.e., there should minimal reflections from the negative delay side ofthe zero delay). By contrast, in FIG. 7A, image 708 represents an imageof the tissue, which can be determined because the zero delay is setjust outside the imaging probe wall, and consequently, the positivedelay space is disambiguated from the negative delay space.

Image 710 is a cropped version of image 704, which can be used to removea portion of image 710 that overlaps with image 706, while image 712 isa version of image 708 with a number of (blank) lines corresponding tothe spatial shift added to the bottom of the image.

FIG. 7B shows additional examples of images representing variousoperations that can be performed during synthesis of image datagenerated using different reference arms in accordance with someembodiments of the disclosed subject matter. Images 712 and 710 arereproduced in FIG. 7B for reference. As shown in FIG. 7B, a binary maskcan be generated for the portion of image 710 that overlaps with zeropadded image 712 based on the mirror image of the tissue in image 706.

As shown in FIG. 7B, a merged image 724 that was generated by combiningimages 710 and 712 using binary mask 722 and weighting coefficients Cmhas greater sensitivity than the channel 1 image alone (whichcorresponds to an image that can be generated using conventional SD-OCTtechniques), without mirror artifacts that would otherwise be present ifthe zero optical delay were set within the sample.

FIG. 7C shows an example of weighted merging coefficients that can beused to synthesize image data generated using different reference armsin accordance with some embodiments of the disclosed subject matter.

FIG. 8 shows examples of sensitivity roll-off observed for a firstreference arm, a second reference, and a combination of the first andsecond reference arms of a spectral domain optical coherence tomographysystem implemented in accordance with some embodiments of the disclosedsubject matter. As shown in FIG. 8, measured maximum sensitivities of anexample system corresponding to each of the two reference delays wereabout ˜105 dB. More particularly, FIG. 8 shows depth-dependentsensitivity for the two reference arms individually (Ch. 1 and Ch. 2curves) and for the combined data (Merged-linear and Merged-tanh,respectively). The latter two curves show the resultant sensitivityroll-off when linear and (tanh+1)/2 functions, respectively, were usedto generate merging coefficients. As shown, the sensitivity for thefirst arm was ˜105 dB near the zero-delay line, but decayed by about 20dB at the end of the imaging depth range (i.e. ˜6 mm from the zerooptical delay for the example system described below). The delay of theother reference arm was set to be about half of the maximum imagingdepth range (i.e. ˜3 mm away from the zero-delay line for the examplesystem described below), which resulted in a sensitivity of ˜98 dB atthe ˜6 mm scan with a peak sensitivity of ˜105 dB at ˜3 mm. The offsetbetween the two reference arms was chosen such that for an imaging depthof 0-3 mm, reference 1 provided better sensitivity (105 dB to 98 dB) andfor 3-6 mm, reference 2 provided better sensitivity (105 dB to 98 dB).Accordingly, in some embodiments, the mechanisms described herein can beused to generate SD-OCT images with a sensitivity that remains betweenthe 105 dB to 98 dB throughout the imaging depth range. This cangenerate a >10 dB improvement in sensitivity compared to conventionalSD-OCT systems where sensitivity decays from 105 dB to 85 dB (e.g., asshown for Ch. 1 in FIG. 8).

An example system based on example 200 was built to generate examplesensitivities. The example system incorporated a 3 milliwatt (mW)superluminescent diode (SLD) with a 3 dB bandwidth of 100 nanometers(nm), centered at about 1310 nm. A booster optical amplifier (BOA, 87 nmFWHM) was used to amplify light from the SLD. A polarization controllerwas used between the SLD and BOA to optimize light amplification and itsspectral profile, as the light amplification by the BOA that was usedwas polarization sensitive. A 75/25 fiber splitter was used to dividethe light from the BOA into sample and reference arms. In the samplearm, an optical circulator was used to direct light toward the sample,and to collect back-reflected light from the sample. The back-reflectedlight from the sample was split using a 50/50 fiber splitter to directhalf of the back-reflected light toward a first reference arm, and theother half toward a second reference arm. Another 50/50 fiber splitterwas used to split the reference arm light, with one reference arm lengthset such that it was at the zero-delay point with respect to the samplearm (i.e., to have an identical optical path length to the sample arm).A linear translation stage and collimation assembly was used to set thepath length of the second reference arm to have a predetermined, 3 mmoptical path length offset compared to the first reference arm, whichwas about half the ranging depth offered by a 2048-pixel line scancamera used in a spectrometer used in the example system.

Fiber couplers with a splitting ratio of 99/1 were used to combine lightreturning from the evenly split sample and reference arms. The 99%outputs from these couplers were passed to the spectrometer through a2×1 optical switch, with one of the 99/1 coupler outputs connecteddirectly to input port 1 of the optical switch, and the other 99/1coupler output transmitted through a 1.5 km fiber delay line (FDL) priorto being connected to input port 2 of the optical switch. An output portof the optical switch was connected to the spectrometer (a Cobra superSWIR 1310, Wasatch Photonics). The 1.5 km length of the FDL was selectedsuch that a temporal delay was induced that was equivalent to theacquisition time of a single line of data by the 140 kHz line scancamera (7.14 microseconds (μs)) (GL2048 R, Sensors Unlimited) used withthe spectrometer. Based on the input signal to the optical switch, itblocked light returning from channel-2 (Ch-2) while it passed light fromchannel-1 (Ch-1) and vice versa (e.g., as described above in connectionwith FIG. 2A). The isolation efficiency of the optical switch was 20 dB,which could potentially lead to cross-talk interference being detectedin the OCT images. However, the FDL eliminated the detection ofcross-talk interference, as it made the optical path length in onechannel significantly longer than that of the ranging depth of thesystem. Two synchronized TTL signals at 140 kHz and 70 kHz were used totrigger the detection of linear array data (2048 pixels per line scan)and to drive the optical switch. This configuration allowed alternateinterferometric spectra to be acquired by the camera resulting frominterference between the sample (within the same temporal window) andthe two different reference arms. Spectra associated with interferencebetween the sample and each reference arm were digitized sequentiallyand processed individually (e.g., as described above in connection withFIGS. 6 and 7). The processing steps included background subtraction,resampling, fast Fourier transformation, and algorithmic mixing of theA-lines to generate a single OCT image with a decreased sensitivityroll-off

A final, mixed image was generated by merging image-1 (imagecorresponding to Ref-1 and Ch-1) with image-2 (image corresponding toRef-2 and Ch-2). However, due to positive vs. negative distanceambiguity resulting from the Fourier transform of real data, the tissuepresent within 3 mm of the imaging probe's surface could lead tomirror-image artifacts in image-2. Mirror artifacts were mitigated inthe combined image, by removes mirror artifacts while merging the twoimages to generate final image having enhanced sensitivity for deeperportions of the ranging depth and increase the imaging range of thesystem than that of standard SDOCT system.

Excised swine colon tissue was used to demonstrate the capacity of themechanisms described herein to acquire high quality image data at depth.An 11-mm diameter tethered OCT capsule was used as a scanning probe forgenerating the image data. The imaging capsule probe was placed in thelumen of the intact swine colon, and circumferential images of theintestinal wall were acquired. Images with single and dual referencearms were acquired for comparison.

Images 702 and 704 depict the OCT images taken by the capsule probe fromexcised swine colon tissue following Ch1 and Ch2. Images 706 and 708 arethe images split from image 702, to separate real and mirror images.Image 722 is a binary mask obtained from image 706, with the binary maskbeing used to remove the overlapped mirror artifact from image 704. Theoptical delay offset between the images 702 and 704 was computed using across-correlation scheme. Image 710 depicts a cropped version of image704 that shows only the positive optical delay with respect to the Ch1image 702. Image 712 shows a zero-padded version of image 708 to matchthe dimensions of FIG. 710 before merging. FIG. 7C shows weightedmerging coefficients calculated from binary mask 722 and the mergingfunction (in this case the merging function as (tanh+1)/2 was used togradually change the weight for merging coefficient from image 1 toimage 2). Image 724 shows a final merged image. As shown in FIGS. 7A and7B, the OCT image intensity in image 702 generated using only data fromCh-1 was stronger for tissue close to the imaging capsule probe'ssurface, whereas the OCT image intensity from tissue further from thecapsule's surface is lower due to sensitivity roll-off. As can beexpected, the deeper region in image 702 shows very dull imageintensity. By contrast, image 704 shows better signal strength fortissues located longer distance from the surface of the tissue due tothe additional delay in Ch2. These characteristics were combined usingthe mechanisms described herein to generate merged image 724, whichshows relatively strong sensitivity throughout, facilitating improvedvisualization of the lumen wall at all depths within the maximum depthof the SD-OCT system. The improvement afforded by the dual referenceSD-OCT system can also be clearly seen by comparing scan convertedsingle- and dual-reference images (e.g., images 308 and 310).

FIG. 9 shows an example 900 of hardware that can be used to implement animaging device and/or a computing device that can be used in connectionwith some embodiments of mechanisms for multiple reference arm spectraldomain optical coherence tomography implemented in accordance with someembodiments of the disclosed subject matter. For example, hardware shownin FIG. 9 can be used to implement at least a portion of spectrometer124. As shown in FIG. 9, in some embodiments, an imaging system 910 caninclude a hardware processor 912, a user interface and/or display 914,one or more communication systems 918, memory 920, one or more lightsources 922, one or more electromagnetic detectors 926, and/or one ormore optical connectors 926. In some embodiments, hardware processor 912can be any suitable hardware processor or combination of processors,such as a central processing unit (CPU), a graphics processing unit(GPU), a microcontroller (MCU), a field programmable gate array (FPGA),a dedicated image processor, etc. In some embodiments, input(s) and/ordisplay 914 can include any suitable display device(s), such as acomputer monitor, a touchscreen, a television, a transparent orsemitransparent display, a head mounted display, etc., and/or inputdevices and/or sensors that can be used to receive user input, such as akeyboard, a mouse, a touchscreen, a microphone, a gaze tracking system,motion sensors, etc.

In some embodiments, communications systems 918 can include any suitablehardware, firmware, and/or software for communicating information over acommunication network 902 and/or any other suitable communicationnetworks. For example, communications systems 918 can include one ormore transceivers, one or more communication chips and/or chip sets,etc. In a more particular example, communications systems 918 caninclude hardware, firmware and/or software that can be used to establisha Wi-Fi connection, a Bluetooth connection, a cellular connection, anEthernet connection, an optical connection, etc.

In some embodiments, communication network 902 can be any suitablecommunication network or combination of communication networks. Forexample, communication network 902 can include a Wi-Fi network (whichcan include one or more wireless routers, one or more switches, etc.), apeer-to-peer network (e.g., a Bluetooth network), a cellular network(e.g., a 3G network, a 4G network, etc., complying with any suitablestandard, such as CDMA, GSM, LTE, LTE Advanced, WiMAX, etc.), a wirednetwork, etc. In some embodiments, communication network 902 can be alocal area network, a wide area network, a public network (e.g., theInternet), a private or semi-private network (e.g., a corporate oruniversity intranet), any other suitable type of network, or anysuitable combination of networks. Communications links shown in FIG. 9can each be any suitable communications link or combination ofcommunications links, such as wired links, fiber optic links, Wi-Filinks, Bluetooth links, cellular links, etc.

In some embodiments, memory 920 can include any suitable storage deviceor devices that can be used to store instructions, values, etc., thatcan be used, for example, by hardware processor 912 to process imagedata generated by one or more optical detectors, to present contentusing input(s)/display 914, to communicate with a computing device 930via communications system(s) 918, etc. Memory 920 can include anysuitable volatile memory, non-volatile memory, storage, any othersuitable type of storage medium, or any suitable combination thereof.For example, memory 920 can include RAM, ROM, EEPROM, one or more flashdrives, one or more hard disks, one or more solid state drives, one ormore optical drives, etc. In some embodiments, memory 920 can haveencoded thereon a computer program for controlling operation of imagingsystem 910. In some such embodiments, hardware processor 912 can executeat least a portion of the computer program to control one or more lightsources and/or detectors (e.g., to capture OCT data as described abovein connection with FIG. 4), to generate images and/or calculate values(e.g., an OCT image, etc.), transmit and/or receive information to/fromcomputing device 930, combine OCT images from different channels togenerate merged OCT images with improved sensitivity roll-off (e.g., asdescribed above in connection with FIG. 6), etc.

In some embodiments, imaging system 910 can include one or more lightsources 922, such a coherent or incoherent light source (e.g., a lightemitting diode or combination of light emitting diodes, a white lightsource, etc.), which can be a broadband light source, or a narrower bandlight source. For example, the bandwidth of the light source can beselected to provide a range of wavelengths that facilitates depthdetection over a maximum imaging range of the SD-OCT system.Additionally, in some embodiments, light sources 922 can be associatedwith one or more filters.

In some embodiments, imaging system 910 can include one or more lightdetectors 924, such as one or more photodiodes, and/or one or more imagesensors (e.g., a CCD image sensor or a CMOS image sensor, either ofwhich may be a linear array or a two-dimensional array). For example, insome embodiments, detectors 924 can include one or more detectorsconfigured to detect light at specific wavelengths (e.g., using filters,using optics to guide light of different wavelengths to differentportions of the detector(s), etc.)

In some embodiments, imaging system 910 can include one or more opticalconnectors 926. For example, such optical connectors can be fiber opticconnectors configured to form an optical connection between lightsource(s) 922 and/or detector 924 and an optical fiber (e.g., as part ofa fiber optic cable).

In some embodiments, computing device 930 can include a hardwareprocessor 932, a display 934, one or more inputs 936, one or morecommunication systems 938, and/or memory 940. In some embodiments,hardware processor 932 can be any suitable hardware processor orcombination of processors, such as a CPU, a GPU, an MCU, an FPGA, adedicated image processor, etc. In some embodiments, display 934 caninclude any suitable display devices, such as a computer monitor, atouchscreen, a television, a transparent or semitransparent display, ahead mounted display, etc. In some embodiments, inputs 936 can includeany suitable input devices and/or sensors that can be used to receiveuser input, such as a keyboard, a mouse, a touchscreen, a microphone, agaze tracking system, motion sensors, etc.

In some embodiments, communications systems 938 can include any suitablehardware, firmware, and/or software for communicating information overcommunication network 902 and/or any other suitable communicationnetworks. For example, communications systems 938 can include one ormore transceivers, one or more communication chips and/or chip sets,etc. In a more particular example, communications systems 938 caninclude hardware, firmware and/or software that can be used to establisha Wi-Fi connection, a Bluetooth connection, a cellular connection, anEthernet connection, etc.

In some embodiments, memory 940 can include any suitable storage deviceor devices that can be used to store instructions, values, etc., thatcan be used, for example, by hardware processor 932 to present contentusing display 934, to communication with one or more imaging devices,etc. Memory 940 can include any suitable volatile memory, non-volatilememory, storage, any other suitable type of storage medium, or anysuitable combination thereof. For example, memory 940 can include RAM,ROM, EEPROM, one or more flash drives, one or more hard disks, one ormore solid state drives, one or more optical drives, etc. In someembodiments, memory 940 can have encoded thereon a computer program forcontrolling operation of computing device 930. In such embodiments,hardware processor 932 can execute at least a portion of the computerprogram to receive content (e.g., image content) from one or moreimaging devices (e.g., imaging device 910), combine OCT images fromdifferent channels to generate merged OCT images with improvedsensitivity roll-off (e.g., as described above in connection with FIG.6), present content (e.g., images and/or values,) transmit content toone or more other computing devices and/or imaging systems, etc.

In some embodiments, computing device 930 can be any suitable computingdevice, such as a general purpose computer or special purpose computer.For example, in some embodiments, computing device 930 can be asmartphone, a wearable computer, a tablet computer, a laptop computer, apersonal computer, a server, etc. As another example, in someembodiments, computing device 930 can be a medical device, a systemcontroller, etc.

In some embodiments, any suitable computer readable media can be usedfor storing instructions for performing the functions and/or processesdescribed herein. For example, in some embodiments, computer readablemedia can be transitory or non-transitory. For example, non-transitorycomputer readable media can include media such as magnetic media (suchas hard disks, floppy disks, etc.), optical media (such as compactdiscs, digital video discs, Blu-ray discs, etc.), semiconductor media(such as RAM, Flash memory, electrically programmable read only memory(EPROM), electrically erasable programmable read only memory (EEPROM),etc.), any suitable media that is not fleeting or devoid of anysemblance of permanence during transmission, and/or any suitabletangible media. As another example, transitory computer readable mediacan include signals on networks, in wires, conductors, optical fibers,circuits, any other suitable media that is fleeting and devoid of anysemblance of permanence during transmission, and/or any suitableintangible media.

It will be appreciated by those skilled in the art that while thedisclosed subject matter has been described above in connection withparticular embodiments and examples, the invention is not necessarily solimited, and that numerous other embodiments, examples, uses,modifications and departures from the embodiments, examples and uses areintended to be encompassed by the claims attached hereto. The entiredisclosure of each patent and publication cited herein is herebyincorporated by reference, as if each such patent or publication wereindividually incorporated by reference herein.

Various features and advantages of the invention are set forth in thefollowing claims.

1. A system for spectral domain optical coherence tomography,comprising: a light source; an image sensor; a sample arm coupled to thelight source, wherein the sample arm is configured to cause light fromthe light source to be projected toward a sample; a first reference amihaving a first path length, wherein the first reference arm is coupledto the light source and the sample arm; a second reference arm having asecond path length that is longer than the first path length, whereinthe second reference arm is coupled to the light source and the samplearm; a first optical coupler configured to combine light from the samplearm and light from the first reference arm; a second optical couplerconfigured to combine light from the sample arm and light from thesecond reference ami; and an optical switch comprising: a first portcoupled to an output of the first optical coupler, a second port coupledto an output of the second optical coupler via a length of opticalwaveguide that induces a delay at least equal to an acquisition time ofthe image sensor, and a third port coupled to the image sensor, whereinthe optical switch is configured to selectively provide light from oneof the first port and the second port and block light from the other ofthe first port and the second port.
 2. The system of claim 1, furthercomprising: a processor that is programmed to: cause the optical switchto output light received by the first port during a first time window,cause the image sensor to generate a first vector of data based on lightreceived during the first time window, wherein the first vector of datarepresents a state of the sample in the first time window that includesa first time, cause the optical switch to output light received by thesecond port during a second time window, cause the image sensor togenerate a second vector of data based on light received during thesecond time window that includes a second time, wherein the secondvector of data represents the state of the sample in the first timewindow, cause the optical switch to output light received by the firstport during a third time window, wherein the third time window issubsequent to the first time window and second time window and includesneither the first time nor the second time, and cause the image sensorto generate a third vector of data based on light received during thethird time window, wherein the third vector of data represents a stateof the sample in the third time window.
 3. The system of claim 2,wherein the first vector and the second vector correspond to a firstlateral position on a surface of the sample, and wherein the thirdvector corresponds to a second lateral position on the surface of thesample.
 4. The system of claim 2, wherein the processor is furtherprogrammed to generate a matrix of data, wherein the first vector is afirst row of the matrix, the second vector is a second row of thematrix, and the third vector is a third row of the matrix.
 5. The systemof claim 4, wherein the first vector comprises at least N elements,wherein each of the N elements corresponds to a pixel of the imagesensor, and wherein the matrix comprises at least N columns.
 6. Thesystem of claim 1, wherein a second optical path from the second opticalcoupler to the second port of the optical switch is longer than a firstoptical path from the first optical coupler to the first port of theoptical switch by at least five light-microseconds.
 7. The system ofclaim 6, wherein the second optical path from the second optical couplerto the second port of the optical switch is at least one kilometer long.8. The system of claim 1, further comprising a fiber splitter with aninput coupled to the light source, a first output coupled to the samplearm, and a second output coupled to the first reference ann and thesecond reference arm, wherein the fiber splitter is configured toprovide at least half of the light received at the input to the firstoutput.
 9. The system of claim 8, wherein the fiber splitter isconfigured to provide three quarters of the light received at the inputto the first output.
 10. The system of claim 8, wherein the fibersplitter is a first fiber splitter, and the system further comprising asecond fiber splitter with an input coupled to the second output of thefirst fiber splitter, a first output coupled to the first reference arm,and a second output coupled to the second reference arm, wherein thesecond fiber splitter is configured to provide half of the lightreceived at the input to the first output.
 11. The system of claim 1,further comprising: a first optical circulator coupled to the lightsource, the sample arm, the first optical coupler, and the secondoptical coupler; a second optical circulator coupled to the lightsource, the first reference arm, and the first optical coupler; and athird optical circulator coupled to the light source, the secondreference arm, and the second optical coupler.
 12. The system of claim1, further comprising a variable delay line coupled between the lightsource and the first reference arm.
 13. The system of claim 1, wherein asensitivity of the system is at least 98 dB over the entire imagingdepth of the system.
 14. The system of claim 1, wherein the second pathlength is longer than the first path length by half of the maximumimaging depth of the system. 15-26. (canceled)